Optical communications systems typically include a pair of network nodes connected by an optical waveguide (i.e., fiber) link. Within each network node, communications signals are converted into electrical signals for signal regeneration and/or routing, and converted into optical signals for transmission through an optical link to another node. The optical link between the network nodes is typically made up of multiple concatenated optical components, including one or more (and possibly 20 or more) optical fiber spans (e.g., of 40-150 km in length) interconnected by optical amplifiers.
The use of concatenated optical components within a link enables improved signal reach (that is, the distance that an optical signal can be conveyed before being reconverted into electrical form for regeneration). Thus, for example, optical signals are progressively attenuated as they propagate through a span, and amplified by an optical amplifier (e.g., an Erbium Doped Fiber Amplifier—EDFA) prior to being launched into the next span. However, signal degradation due to noise and dispersion effects increase as the signal propagates through the fiber. Consequently, noise and dispersion degradation become significant limiting factors of the maximum possible signal reach.
Dispersion, also known as Group Velocity Dispersion or Chromatic Dispersion, in single mode fibre at least, occurs as a result of two mechanisms:                a) Waveguide dispersion—within a single mode fibre, different wavelengths travel along the fibre at different speeds; and        b) Material dispersion—the phase velocity of plane waves in glass varies with wavelength.        
For the purposes of the present invention, references to “dispersion” shall be understood to mean the sum total of group velocity dispersion effects.
Mathematically, first order dispersion is the derivative of the time delay of the optical path with respect to wavelength. The effect of dispersion is measured in picoseconds arrival time spread per nanometre line width per kilometer length (ps·nm−1·km−1). The magnitude of waveguide and material dispersions both vary with wavelength, and at some wavelengths the two effects act in opposite senses. The amount of dispersion present in a link can also vary with the temperature of the cable, and if the route is changed (e.g., using optical switches). Dispersion in optical fibre presents serious problems when using light sources whose spectrum is non-ideal, for example broad or multispectral-line, or when high data rates are required, e.g., over 2 GB/s.
A known method of compensating dispersion is to produce an optical single sideband (OSSB) signal at the transmitter, typically by means of passive optical filters to suppress the unwanted sideband. FIG. 1a schematically illustrates an OSSB signal 2 in which the filter characteristic 4 used to suppress the unwanted sideband 6 (shown in dashed lines) is shown superimposed on the OSSB signal. At the receiver-end of the link, the OSSB signal is directly detected using a photodiode in a conventional manner, to produce an electrical signal (FIG. 1b) having terms corresponding to the carrier signal 8, the desired sideband 10, and noise 12. The carrier and sideband terms 8 and 10 contain the original phase information (and thus the transmitted data) of the inbound OSSB signal 2. Provided that the power level of the noise term 12 is low enough, this noise term can be removed by filtering, and accurate compensation of dispersion can be obtained by filtering the residual electrical signal using a transversal filter having a linear group delay response. This technique is disclosed in, for example, Demonstration of Electrical Dispersion Compensation of Single Sideband Optical Transmission, P. M. Watts et al., University College London; 10 Gbit/s 177 km transmission over conventional singlemode fiber using a vestigial side-band modulation format, Hoon Kim and A. H. Gnauck, Electronics Letters, Vol. 37, No. 25, pp 1533-1534, 6 Dec. 2001; Dispersion Mitigation Using a Fiber-Bragg-Grating Sideband Filter and a Tunable Electronic Equalizer, H Bulow et al, Optical Society of America, 2000; and 10Gbit/s Optical Single Sideband System, M. Sieben et al, Electronics Letters, Vol. 33, No. 11, pp 971-973, 22 May 1997.
A limitation of the above-described techniques is that real optical filters are imperfect, with the result that it is extremely difficult to produce a “pure” single sideband optical signal at the transmitter. In practice, at least some optical power remains in the unwanted sideband 6, resulting in the transmission of a “vestigial” sideband 6a, as shown in FIG. 1a. At the receiver-end of the link, the vestigial sideband 6a is convolved with both the optical carrier and the wanted sideband, and results in phase-distortions in the electrical signal generated by the photodiode. This effectively increases the noise term 12 in the electrical signal (FIG. 1b), and reduces the amount of dispersion that can be compensated by electrical filtering.
An alternative method of generating an OSSB signal is to sequentially drive amplitude and phase modulations of the carrier signal using phase-shifted versions of the same drive signal. Both modulations generate a pair of sidebands, which combine additively on one side of the carrier and subtractively on the other. When the amplitude and phase modulations are perfectly matched, one sideband is cancelled resulting in a “pure” OSSB signal. A mathematically equivalent approach is to use a dual-branch Mach-Zehnder (MZ) modulator, in which each branch is driven by a respective 90° phase-shifted version of a common drive signal. Both of these techniques are described by Sieben, M et al. (supra). U.S. Pat. No. 5,301,058 (Olshansky), which issued on Apr. 4, 1994, describes methods and apparatus for generating an OSSB signal using a dual-branch MZ modulator.
However, successful implementation of either of these technique relies upon perfect matching of the drive signals, electrical signal paths, and optical components. Normal manufacturing variations effectively preclude such a perfect match. Even if an “ideal” MZ modulator and a “perfect” Hilbert Transformer are obtained, the non-linear response of the MZ modulator means that a vestigial sideband will remain, at least for large signal operation. As a result, complete cancellation of the unwanted sideband is practically unobtainable, with the result that these techniques offer little or no performance improvement over passive filtering. Thus, for example, Sieben et al (supra) report simulation results suggesting that 10 Gbit/s OSSB signals can be transmitted over distances of >1000 km with post-detection electrical dispersion compensation. However, they report that transmission distances of only 200 km were achieved in actual experimental trials.
Accordingly, methods and apparatus for cost-effectively generating readily compensated optical signals remains highly desirable.